The present invention is directed to a method of removing contaminants from a double-ended arc discharge tube and to a configuration of the arc discharge tube during manufacture.
With reference to FIG. 1, a conventional arc lamp 10 includes a double-ended arc discharge tube 12 with electrodes 14 and 16 sealed in opposite ends of tube 12. Foils 18 and 20 in press seal regions 22 and 24 electrically connect electrodes 14 and 16 to external leads 26 and 28. Tubulation 30 is attached to an opening in a side of tube 12 that leads to discharge region 32. The interior surface of discharge region 32 and electrodes 14 and 16 contain contaminants that should be removed to improve lamp performance. The contaminants are removed with a flushing gas that is fed into discharge region 32 and then removed, carrying away the contaminants. Tubulation 30 includes an inner needle 34 through which a flushing gas is introduced into region 32 and an annular portion around needle 34 through which the flushing gas and contaminants are removed. This lamp is disclosed in U.S. Pat. No. 5,176,558 that is incorporated by reference.
As is apparent from FIG. 1, the inlets and outlets for the flushing gas are very close to each other at one side of tube 12. This is necessary because tubulation 30 is desirably small to avoid a large hole in the side of tube 12 that must be closed later. The closure of such a hole is accomplished with a tip-off that can undesirably distort the side of tube 12 and is a cold spot during lamp operation that degrades lamp color and uniformity of emitted light.
However, the small tubulation hole forces the inlet and outlet for the flushing gas close to each on one side of tube 12, and the cleaning action of the flushing gas is reduced. Some areas of the interior of tube 12 receive less flow and contaminants may remain in such areas. Further, flushing gas may be wasted because the close proximity of the inlet and outlet may allow clean flushing gas to be immediately drawn through the outlet before it has been flushed through the interior of tube 12. A more robust and economical cleaning action, preferably without the tip-off, is desirable.
One technique for introducing a flushing gas without a tip-off is disclosed in U.S. Pat. No. 5,037,342 that is also incorporated by reference. This patent relates to a single-ended arc discharge tube that includes a removable pipe in the sealed end through which gases and materials are introduced into the arc discharge region. Flushing gas can be provided through the removable pipe and thus the tube does not require a tip-off.
However, the flushing gas is introduced and removed from the same orifice and thus the removable tube affords the same, less rigorous, cleaning action noted above because the flushing gas does not flow generally uniformly throughout the interior of the tube. Further, the insertion and removal of the pipe adds steps and complexity to the process.
In a further embodiment of this patent in which the arc discharge tube is placed in an outer envelope that includes only a gas fill, two capillaries are provided in the sealed end of the outer envelope. However, the two capillaries are close to each other in one end of the outer envelope and cannot be used for thorough cleaning. Indeed, the patent recognizes this shortcoming and states that the two capillaries are useful where only a fill gas is to be introduced and the need for high purity is less important.
An object of the present invention is to provide a novel method for flushing contaminants from a double-ended arc discharge tube that offers rigorous and economical cleaning action without forming a tip-off on the arc discharge tube.
A further object of the present invention is to provide a novel method of removing contaminants from a double-ended arc discharge tube that includes the steps of providing at least one capillary channel at each end of the tube, where the ends of the tube are sealed closed except at the capillary channels, and introducing a flushing gas into the tube through at least one capillary channel at one end of the tube and removing the flushing gas and contaminants through one or more capillary channels at another end of the tube.
A yet further object of the present invention is to provide a double-ended arc discharge tube that, during manufacture, has a sealed electrode and one or more capillary channels at each end of the arc discharge tube.
Another object of the present invention is to provide a novel method of making a double-ended arc discharge tube, that includes the steps of providing a cylinder of light transmissive material, inserting electrodes into each end of the cylinder, pressing the light transmissive material to seal the electrodes and form an unfilled double-ended arc discharge tube while at each of the pressed ends leaving open at least one capillary channel, removing contaminants from the tube by flushing a gas lengthwise through the tube using the capillary channels at both ends of the tube, introducing a fill gas and lamp chemicals into the tube using at least one of the capillary channels, and closing the capillary channels.